The present invention relates to envelopes for electrical conductors and cable, and made of thermoplastic or elastomeric material, and more particularly the invention relates to the making of such thermoplastic or elastomeric envelopes for cables and conductors which will be used for conduction of higher electrical voltages. The invention will find utility for crosslinked plastic materials as well as for plastics without crosslinking, but with predominant interest for thick envelopes.
Conductors and cables are quite frequently insulated in that a thermoplastic or elastomeric material is extruded onto the conductive cable core. In some instances crosslinking of the plastic is needed simply because the electrical transmission characteristic of the cable requires that kind of insulation. Therefore, the cable has to be heated subsequently to obtain crosslinking. If the cable is to be used for high voltage, the insulation may have to be more than 10mm thick. It has not yet been possible to provide for an adequate method of enveloping a conductor with such a thick envelope and in a horizontal production line, because the insulation will flow around the conductor by force of gravity and form a drop; on a continuous basis, this amounts to an excentric displacement of the conductor in the envelope so that the wall thickness of the insulation is not only not uniform but is locally too thin.
Aside from these drawbacks as resulting from the geometry, additional problems arise pursuant to cooling, because internal strain (resulting from fast surface cooling) will inevitably produce local weak spots as far as insulative strength in concerned. An envelope which is asymmetric in cross-section must be expected to have an asymmetric stress pattern which may be result in excessive local stress on bending.
In order to avoid the aforementioned deficiencies it has been suggested to insulate high voltage cable e.g., with polyethylene in a tower like production line. The conductor to be enveloped is run up in a 50 to 60m high tower, reverses its direction, and the envelope is extruded on its down path. Crosslinking is obtained right behind the extruder in a so called vulcanizing station. The enveloped conductor then runs into a water filled pipe for cooling. Drop formation of the insulation is simply avoided by the vertical orientation of the conductor when enveloped with more or less liquidous, but also vertically oriented plastic envelope.
The problem posed by an excentrically arranged conductor in an envelope is indeed avoided by such vertical production line. However, to construct a 200 foot high tower for that purpose (and a manufacturing hall to put the tower in) is rather undesirable to say the least. Additionally, the production speed is quite limited when enveloping the conductor along a vertical line.